Particle nucleation mechanisms

Achieving steady-state operation in a continuous tank reactor system can be difficult. Particle nucleation phenomena and the decrease in termination rate caused by high viscosity within the particles (gel effect) can contribute to significant reactor instabilities. Variation in the level of inhibitors in the feed streams can also cause reactor control problems. Conversion oscillations have been observed with many different monomers. These oscillations often result from a limit cycle behavior of the particle nucleation mechanism. Such oscillations are difficult to tolerate in commercial systems. They can cause uneven heat loads and significant transients in free emulsifier concentration thus potentially causing flocculation and the formation of wall polymer. This problem may be one of the most difficult to handle in the development of commercial continuous processes. 

In mini-emulsion polymerization, the particle nucleation mechanism may be evaluated by the ratio of the final number of polymer particles to the initial number of monomer droplets (Np f/Nm i). If the particle nucleation process is primarily governed by entry of radicals into the droplets, then the value of Np>f/Nm>i should be around 1. A lower value of Np f/Nm i may imply incomplete droplet nucleation or coalescence. On the other hand, a higher value of Npf/Nm>i may indicate that the influence of micellar or homogeneous nucleation comes into play in the particle formation process, since one droplet feeds monomer to more than one micelle in the classical emulsion polymerization. For pure micel-... 

It was found that the SDS/CA containing polymerization system shows a bi-modal PSD of latex particles, and the SDS/DMA containing system is characterized by a quite broad PSD. The fraction of small latex particles results from the initially generated highly monomer-swollen polymer particles, which serve as the monomer reservoir in the later stage of polymerization. On the contrary, the latex products obtained from both the SDS/SMA and SDS/HD containing systems show a relatively narrow PSD. These data further support the proposed competitive particle nucleation mechanism. 

Formation of latex particles can proceed via the micellar nucleation, homogeneous nucleation and monomer droplet nucleation. The contribution of each particle nucleation mechanism to the whole particle formation process is a complex function of the reaction conditions and the type of reactants. There are various direct and indirect approaches to determine the particle nucleation mechanism involved. These include the variations of the kinetic, colloidal and molecular weight parameters with the concentration and type of initiator and emulsifier. There are some other approaches, such as the dye method where the latex particles generated via homogeneous nucleation do not contribute to the amount of dye detected in the latex particles since diffusion of the extremely hydrophobic dye molecules from the monomer droplets to the latex particles generated in water is prohibited. On the contrary, nucleation of the dye containing monomer droplets leads to the direct incorporation of dye into the polymer product. However, the dye also act as a hydrophobe and enhances the stability of monomer droplets as well as the monomer droplet nucleation. 

A polymerization with n very large (102-6) indicating suspension-like (bulk polymerization) kinetic behavior, and a particle nucleation mechanism residing outside the monomer droplets, which delineates an emulsion process. Smith-Ewart case III systems are examples of this type of behavior provided they have evolved from case I and/or case II polymerization at low conversions, which is common. 

Emulsions and suspensions can be distinguished by the particle nucleation mechanism and the kinetics. A polymerization is considered an emulsion if... 

As explained before, when surfactant, water, and monomer(s) are mixed, the colloidal system obtained consists of monomer-swollen micelles (if the surfactant concentration exceeds its CMC) and monomer droplets dispersed in an aqueous phase that contains dissolved molecules of surfactant and a small amount of the sparingly water-soluble monomer(s). When free radicals are generated in the aqueous phase by action of an initiator system, then the emulsion polymerization takes place. Its evolution is such that the colloidal entities initially present tend to disappear and new colloidal entities (polymer latex particles) are bom by a process called nucleation. For convenience, we first focus on the particle nucleation mechanisms, a very important aspect of emulsion polymerization. 

One of the outstanding issues in this area is the need for reliable experimental data to quantify the contribution of each one of the nucleation mechanisms to particle formation in emulsion polymerization. Also outstanding is the development of a complete mathematical model which combines the contributions of all particle nucleation mechanisms. 

A continuous particle nucleation mechanism was further confirmed by transmission electron microscopic (TEM) experiments performed on polyacrylamide samples taken at various degrees of conversion [25]. The number of polymer particles was shown to increase proportionally with conversion (Fig. 6a), whereas the size remained roughly constant. 

The mechanism of polymerization in ternary and quaternary oil-in-water microemulsions has become understood only in recent years. The onset of turbidity upon polymerization and the lack of stability with time observed by most authors, particularly for MM A monomer, is likely the reason for the slow progress in the comprehension of the mechanism of O/W systems. Only slight changes in the formulation are sufficient to significantly affect the polymerization process and to induce particle coagulation at any stage of the reaction. This may explain the disparity in the kinetic data reported by some authors for very similar systems. With this remark in mind, one can, however, conclude that the scheme that is now well accepted is that of a continuous particle nucleation mechanism as in the case of inverse systems. This view is supported by several features. 

Oil-in-water emulsion polymerization systems are typically classified as possessing the characteristics of one of three types of emulsions macro-emulsions, mini-emulsions or microemulsions. These emulsions are the initial systems for emulsion polymerization. There are quite differences between these systems in some aspects such as the size of the droplets (i.e. the discontinuous or dispersed phase), the interfacial area of the droplets, the particle nucleation mechanism and the stability of the emulsion. 

The technique of pyrene fluorescence intensity measurements was proposed to study the particle nucleation mechanisms involved in the 0/W microemulsion polymerization (31). The experimental data showed that microemulsion droplets are the major particle nucleation loci for the polymerization system with the more hydrophobic ST as the monomer. This is followed by the flocculation of latex particles with the remaining droplets. In contrast, the polymer reactions taking place initially in the continuous aqueous phase (homogeneous nucleation) plays an important role in the MMA microemulsion polymerization. 

The kinetic models presented earlier are based on steady-state operation. Unfortunately, achieving a stable steady state can be a difficult problem for CSTR systems. Sustained, limit-cycle oscillations have been observed by a large number of workers in both academic laboratories and in commercial units. Multiple steady states have also been reported. These phenomena are believed to be caused primarily by the particle nucleation mechanisms with perhaps the gel-effect also being partially responsible. 

For conciseness, this chapter primarily deals with three well-established particle nucleation mechanisms (i.e., micellar nucleation, homogeneous nucleation and coagulative nucleation). This is followed by the discussion of emulsion polymerization kinetics in Chapter 4. 

Based on this particle nucleation mechanism, the mole balance of free radicals in the continuous aqueous phase and the rate of particle nucleation can be described by the following equations. 

Poehlein [40] summarized previous work and proposed a comprehensive particle nucleation mechanism involved in a persulfate initiated emulsion polymerization system, as shown schematically in Figure 3.5. Song and Poehlein [41, 42] developed a general kinetic model taking into account micellar nucleation, homogeneous nucleation, and monomer droplet nucleation in emulsion polymerization. The chain transfer and termination reactions occurring in the continuous aqueous phase, capture of oligomeric radicals by particle nuclei, and flocculation of particle nuclei were also incorporated into the model development. The resultant expressions for calculation of the rate of particle nucleation can be written as... 

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